Add unit tests for Customizer setting sanitize callbacks.
Use a semicolon for the Customizer add_setting() regex.

If a closing parenthesis exists before the 'sanitize_callback' arg, for
instance, when a 'default' argument value is an array, the check will
fail. See the CustomizerTest::testValidSanitizeCallback() unit test
method for an example.

Instead, grab the entire add_setting() call by using a semicolon for the
terminating character.
Update regex for empty sanitize callback check to only look for empty values.

The sanitize callback method can be various 'callable' types, which
include closures or arrays when referencing class methods. The existing
check fails for anything that isn't a string.

Instead, this update checks for any empty string before returning an
error.
Merge pull request #136 from bradyvercher/fix-customizer-checks

Add unit tests and fix Customizer sanitize callback checks.